A computer network is a collection of interconnected computing devices that exchange data and share resources. In a packet-based network, such as the Internet, computing devices communicate data by dividing the data into small blocks called packets. The packets are individually routed across the network from a source device to a destination device. The destination device extracts the data from the packets and assembles the data into its original form. Dividing the data into packets enables the source device to resend only those individual packets that may be lost during transmission. Computing devices may also divide data into other types of data units, such as cells or frames.
Networks that primarily utilize data link layer devices are often referred to as layer two (L2) networks. A data link layer device is a device that operates within the second layer of the Open Systems Interconnection (OSI) reference model, i.e., the data link layer. One example of a common L2 network is an Ethernet network in which end point devices (e.g., servers, printers, computers) are connected by one or more Ethernet switches. The Ethernet switches forward Ethernet frames, also referred to as L2 communications or L2 packets to devices within the network. As the Ethernet switches forward the Ethernet frames, the Ethernet switches learn L2 state information for the L2 network, including media access control (MAC) addressing information for the devices within the network and the physical ports through which the devices are reachable. The Ethernet switches typically store the MAC addressing information in MAC tables. When forwarding an individual Ethernet frame, an ingress port of an Ethernet switch typically broadcasts the Ethernet frame to all of the other physical ports of the switch unless the Ethernet switch has learned the specific physical port through which the destination MAC address devices is reachable. In this case, the Ethernet switch forwards a single copy of the Ethernet frame from the associated physical port.
Computing devices may be interconnected by one or more links. The term “link” is often used to refer to the connection between two devices on a network and may include a physical medium, such as a copper wire, a coaxial cable, or any of a host of different fiber optic lines, or a wireless connection. On occasion, one or more computing devices and/or one or more links of the network may fail due to any of a number of reasons. When a device or link of the network fails, the result is a degradation or loss of service to customers, which is generally undesirable. Administrators of networks often attempt to limit the impact of any such failures, e.g., by limiting the amount of time of the failure.
Operations, Administration and Maintenance (OAM) generally refers to processes, activities, tools, standards, and other techniques that involve operating, administering, and maintaining, e.g., troubleshooting, a computer network. A combination of OAM techniques may constitute an OAM protocol. One such OAM technique, referred to as Connectivity Fault Management (CFM), is described in the Institute of Electrical and Electronics Engineers (IEEE) 802.1 ag standard entitled “IEEE Standard for Local and Metropolitan Area Networks—Virtual Bridged Local Area Networks—Amendment 5: Connectivity Fault Management,” which is hereby incorporated by reference. A network device operating in accordance with CFM may proactively transmit continuity check messages (CCMs) at a predetermined rate to other devices within the same maintenance association, and may receive CCMs from the other devices. A maintenance association is a logical grouping of devices within the network configured to verify the integrity of a single service instance. A service instance may, for example, represent a portion of a provider network that a given customer can access to query a status of services delivered for that customer. The CCMs provide unidirectional connectivity verification to the other network devices within the maintenance association.
The other network devices in the maintenance association may create and maintain a connectivity database of network devices from which periodic CCMs are expected to be received. The network devices may, after establishing connectivity with the other network devices, monitor receipt of CCMs. If a CCM is not received from one of the network devices identified in the connectivity database within a configured time, the network device may identify a fault or a failure, which is commonly referred to as a “connectivity failure.”